Area Forecast Discussion
Issued by NWS St. Louis, MO
Issued by NWS St. Louis, MO
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969 FXUS63 KLSX 111949 AFDLSX Area Forecast Discussion National Weather Service Saint Louis MO 249 PM CDT Mon May 11 2026 .KEY MESSAGES... - There is a 30 to 60 percent chance of showers and thunderstorms Tuesday evening into overnight. A thunderstorm or two could contain gusty winds or small hail. - Temperatures will warm to above average on Tuesday, then cool to near or slightly below average on Wednesday and Thursday. - Warmer temperatures will return Friday into early next week along with opportunities for showers and thunderstorms at times. && .SHORT TERM... (Through Late Tuesday Night) Issued at 248 PM CDT Mon May 11 2026 An elongated surface high pressure center will gradually shift to the southeast overnight tonight with portions of southeastern MO and southwestern IL still seeing efficient radiational cooling with low temperatures in the the upper 40s to around 50 F. Elsewhere, a tightening surface pressure gradient and increasing southeasterly winds will keep low temperatures warmer and in the 50s F. On Tuesday, the Mid-Mississippi River Valley will be positioned in the warm sector of an approaching cold front, with conditions favorable for much warmer, above average temperatures in the upper 70s to mid- 80s F thanks to low-level southwesterly flow/WAA downsloped off the Ozark Plateau and minimal clouds during peak insolation. An upper-level trough tracking across the Upper Midwest and Great Lakes late Tuesday will force a cold front southeastward into the CWA during the evening into overnight. A narrow belt of mid-level moist isentropic ascent near the front will promote development of scattered showers and a few thunderstorms during the evening that migrate southeastward overnight. However, coverage overall is most confident along/east of the Mississippi River where HREF probabilities of measurable rainfall are higher (40 to 60 percent) closer to more vigorous trough-driven ascent passing to the northeast. Strong deep-layer shear of 40 to 45 kt will be present, but instability continues to be a limiting factor for severe thunderstorms. With a lack of sustained Gulf of Mexico moisture fetch including dewpoints only reaching the 50s F and poor mid-level lapse rates owing to a warm layer/capping inversion, the latest HREF only has 20 to 50 percent probabilities of 1000+ J/kg SBCAPE across northeastern MO/west-central IL late Tuesday afternoon where locally higher moisture is pooled along the cold front, quickly decreasing with time and southeastward extent. It is also uncertain if there will be surface-based thunderstorm initiation while instability is greatest, if at all given the capping inversion. Therefore, the predominant expectation is for high-based/elevated showers and thunderstorms to develop across northeastern MO/west-central IL during the evening with a few stronger thunderstorms capable of gusty winds (plentiful sub-cloud dry air) and small hail, decreasing in strength as they migrate eastward/southeastward overnight as instability wanes. Pfahler && .LONG TERM... (Wednesday through Next Monday) Issued at 248 PM CDT Mon May 11 2026 On Wednesday, post-frontal, low-level northerly flow and CAA beneath upper-level northwesterly flow will provide cooler temperatures across the Mid-Mississippi River Valley, but model guidance is in agreement that the core of a seasonably cool post-frontal airmass will remain across the Great Lakes and Upper Ohio Valley. Across the CWA, high temperatures on Wednesday are forecast to range from the upper 60s/around 70 F to the mid-70s F from east to west with a gradient in 850-hPa temperatures. As a surface high pressure center traverses the region late Wednesday through Thursday, CAA will subside and WAA will eventually ensue, but it is uncertain if WAA will begin early enough in the day Thursday to result in much change in high temperatures from Wednesday. Global model guidance vary in their depictions of the upper-level flow pattern evolution late Thursday into early next week with key differences in the amplitude and progression of a potential trough across the Rocky Mountains to Desert Southwest, marking the difference between quasi-zonal flow and more southwesterly flow. Despite differences in exactly how this pattern transpires, there is a general consensus for temperatures to return to above average along with more quality Gulf of Mexico moisture return and passing shortwave troughs leading to showers and thunderstorms at times. Exactly how much temperatures warm will depend on the prevalence and timing of clouds and precipitation, but there continues to be concerns that the NBM rolling bias correction is warming temperatures to the top of the ensemble model guidance distribution and even beyond. This factor skews the current forecast high temperatures based on the NBM to near daily records, which confidence is not high in achieving with 850-hPa temperatures only flirting with the 90th climatological percentile. Confidence is much higher, based on ensemble model guidance, in high temperatures at least largely in the 80s F through the period, potentially cooler if showers and thunderstorms are timed during the day. In terms of precipitation, 40 to 70 percent of ensemble model membership have showers and thunderstorms Thursday night into Friday with a passing shortwave trough and accompanying warm front and decaying cold front. However, other details of this pattern governing precipitation are difficult to discern at this point. Pfahler && .AVIATION... (For the 18z TAFs through 18z Tuesday Afternoon) Issued at 1236 PM CDT Mon May 11 2026 Dry and VFR flight conditions are expected to persist through Tuesday afternoon at all terminals. Patchy river valley fog cannot entirely be ruled out at KSUS tonight, but confidence is low in its occurrence. Otherwise, winds will become southwesterly and strengthen Tuesday morning as a surface high pressure center departs and a cold front approaches from the northwest. Pfahler && .LSX WATCHES/WARNINGS/ADVISORIES... MO...None. IL...None. && $$ WFO LSX